1. Field of the Invention
The present invention relates to a wireless redistribution system for terrestrial digital TV broadcasting.
2. Description of the Related Art
In Japan, the terrestrial digital TV broadcasting started on Dec. 1, 2003 in three metropolitan areas of Kanto, Chukyo and Kinki. The terrestrial digital TV broadcasting adopts an OFDM (Orthogonal Frequency Division Multiplexing) system not being subject to multipath interference, enabling stable high quality reception with little double-reflection of images (ghost defect) which have been caused in the analog TV broadcasting or the like. The terrestrial digital TV broadcasting is performed on the UHF band (470 to 770 MHz).
As modes of receiving the terrestrial digital TV broadcasting with a terrestrial digital TV receiver, generally, there are following types.
(1) Terrestrial digital TV broadcast signals are received by an outdoor antenna. The terrestrial digital TV broadcast signals received by the outdoor antenna are distributed to each room with antenna cables, and guided to a TV terminal provided on a wall of each room. To the TV terminal provided on a wall of each room, an antenna terminal of a terrestrial digital TV receiver is connected via an antenna cable.
In such a reception mode, an antenna terminal of a terrestrial digital TV receiver must be connected to a TV terminal provided on a wall via an antenna cable. This deteriorates the outer appearance and also causes a problem of the wiring being troublesome. Further, layouts of the TV receiver are limited; therefore, the degree of freedom in arrangement positions of the TV receiver is impaired.
(2) An UHF antenna (e.g., UDMF62: manufactured by Nippon Antenna) is provided at an edge of the eaves or veranda without providing an antenna on the rooftop (on the roof), and an antenna cable is drawn into a room therefrom so as to connect directly to an antenna terminal of a terrestrial digital TV receiver.
Even in such a reception mode, an antenna cable must be drawn into a room so as to be connected to a terrestrial digital TV receiver. This also deteriorates the outer appearance and causes a problem of the wiring being troublesome. Further, layouts of the TV receiver are limited; therefore, the degree of freedom in arrangement positions of the TV receiver is impaired.
(3) The terrestrial digital TV broadcast signals are received by an indoor antenna connected to a terrestrial digital TV receiver.
In such a reception mode, limitation in the layouts of the TV receiver caused by wiring of an antenna cable is solved. However, there is a problem that a radio wave shield loss in the room is caused. Particularly, in the UHF band used for terrestrial digital TV broadcast signals, attenuation loss when passing through a building is large comparing with the VHF band used in the terrestrial analog TV broadcasting; therefore, there is a problem that arrangement positions of a TV receiver are limited from the viewpoint of the receiving sensitivity.
Thus, there has been developed an indoor redistribution system in which terrestrial digital TV broadcast signals inputted into a room are amplified by a wireless transmitter, and are retransmitted to a TV receiver wirelessly (see JP-A 2004-128719 and JP-A 2004-128720). In this indoor retransmission system, terrestrial digital TV broadcast signals of the UHF band, inputted into a room, are amplified directly and retransmitted, without being frequency-converted.
In the frequency of the UHF band, a transmission output level of a wireless transmitter is limited to a very low level as described later. This causes a problem that only a short range transmission is possible.
Further, when both of the direct broadcast radio wave transmitted from a broadcasting station and the retransmitted radio wave transmitted from a wireless transmitter are received and inputted in an indoor reception antenna, signals of the same frequency channel are inputted in a TV receiver. Thereby, there may be a case where interference disturbance of the same channel is caused and reception becomes impossible.
In order to prevent such a case, antennas or circuits must be added with devises for preventing deterioration caused by the same channel disturbance of terrestrial digital TV broadcast signals.